This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-331805, filed Oct. 30, 2001; and No. 2002-090388, filed Mar. 28, 2002, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a balanced amplifier and a filter using the same and, more particularly, to a balanced amplifier whose maximum value of an output signal amplitude is larger than that of a conventional amplifier even at a low voltage, and a filter using the same.
2. Description of the Related Art
A balanced amplifier has a gain only for the differential-mode components of an input signal and removes common-mode components. This balanced amplifier can remove noise mixed as common-mode components and double the amplitude of a differential signal as compared with the amplitude of a single-ended signal. Owing to such advantages, the balanced amplifier is widely used in analog-digital mixed integrated circuits and circuits that operate at low voltages. A circuit for removing common-mode components has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 2000-148262) which is configured by a combination of a differential pair and a common-mode feedback (to be referred to as CMFB hereinafter) circuit and removes common-mode components from an input voltage.
In such a conventional circuit, since the common-mode component removing property of the differential pair is used, the number of cascaded transistors which are stacked is limited, and the maximum value of an output signal amplitude is too small when this circuit operates at a low voltage.
For example, in many semiconductor integrated circuits, analog circuits are formed on the same chip on which digital circuits are fabricated. In order to increase the integration degree of circuits, it is advantageous that digital and analog circuits operate at the same voltage. With advances in microprocessing, however, the power supply voltage is further decreased. For example, the operating voltage for a digital circuit in a 0.11-xcexcm process integrated circuit that is expected to be commercialized in the near future is about 1.5V. With a reduction in the size of a process integrated circuit, the operating voltage is expected to further decrease. When the power supply voltage further decreases in this manner, the differential pair cannot exhibit sufficient performance, i.e., a sufficient output amplitude cannot be obtained, when it is operated at the same voltage as that for the digital circuit. Therefore, it may become difficult to obtain a sufficient noise removing effect even if a noise removing circuit is formed by using a balanced amplifier.
It is an object of the present invention to provide a balanced amplifier in which the upper limit of output signal amplitudes is high even at a low voltage and a filter which uses the balanced amplifier and can exhibit sufficient performance even at a low voltage.
According to an aspect of the invention, there is provided a balanced amplifier comprising: a pair of voltage-to-current converters each including a first input terminal, a second input terminal, a first output terminal and a second output terminal, each of the voltage-to-current converters converting differential input voltages applied to the first input terminals of the converters into output currents output from both of the first output terminal and the second output terminal of each of the converters, wherein the second input terminals and the second output terminals of the converters are connected in common to cancel common-mode components and extract differential components.
According to another aspect of the invention, there is provided a balanced amplifier comprising a first voltage-to-current converter and a second voltage-to-current converter, each of the first voltage-to-current converter and the second voltage-to-current converter including a first input terminal, a second input terminal, a first current source which outputs a first current, a second current source which outputs a second current, a first output terminal outputting a third current and a second output terminal outputting a fourth current, wherein the third current is obtained by subtracting a sum current from the first current, the sum current corresponding to sum of currents corresponding to voltages applied to the first input terminal and the second input terminal respectively, and the fourth current being obtained by subtracting the sum current from the second current, and wherein the second input terminal and second output terminal of the first voltage-to-current converter and the second input terminal and second output terminal of the second voltage-to-current converter is connected in common, a differential input signal is input to the first input terminals of the first voltage-to-current converter and the second voltage-current converter, and a differential output signal is output from the first output terminal of the first voltage-to-current converter and the second voltage-current converter.
According to another aspect of the invention, there is provided a balanced amplifier comprising a first voltage-to-current converter and a second voltage-to-current converter, each of the first voltage-to-current converter and the second voltage-to-current converter including a first input terminal, a second input terminal, a first output terminal and a second output terminal, and each supplying a current corresponding to sum of currents corresponding to voltages applied to the first input terminal and the second input terminal respectively to the first output terminal and the second output terminal so that a polarity of an output signal from the first output terminal is reversed with respect to a polarity of an input signal to the first input terminal, and a polarity of an output signal from the second output terminal is reversed with respect to a polarity of an input signal to the second input terminal, wherein the second input terminals and second output terminals of the converters is connected in common, a differential input signal is input to the first input terminals of the converters, and a differential output signal is output from the first output terminals of the converters.
According to another aspect of the invention, there is provided a voltage-to-current converter comprising a first input terminal, a second input terminal, a first output terminal, a second output terminal, an adder which adds voltage signals supplied to the first input terminal and the second input terminal respectively, a first inverting amplifier which reverses and amplifies an output of the adder; a second inverting amplifier which reverses and amplifies the output of the adder, a third inverting amplifier which reverses and amplifies an output of the first inverting amplifier and outputs a first reversed and amplified signal to the first output terminal as a first current signal; a fourth inverting amplifier which reverses and amplifies an output of the second inverting amplifier and outputs a second reversed and amplified signal to the second output terminal as a second current signal; a first capacitor connected between an input terminal and an output terminal of the third inverting amplifier, and a second capacitor connected between an input terminal and an output terminal of the fourth inverting amplifier.